1. Field of the Invention
The present invention relates to an injection molding machine and more particularly to an injection molding machine having a nozzle touch plate that enables a tip opening of a nozzle of a heating cylinder to close and open for enabling a plasticizing and measuring process to be carried out without contacting the nozzle to a fixed die or making it an open state.
2. Description of the Related Art
A process of one molding cycle in an injection molding machine includes a plasticizing and measuring process in which a resin material supplied in a heating cylinder is melted and plasticized by a rotating screw or the like and a predetermined amount of the resin material melted and plasticized is reserved in a front portion of the heating cylinder. The temperature at which the resin material is melted and plasticized changes depending on the resin materials, and the temperature for a die is set according to molding conditions such as the temperature at which the resin material is melted and plasticized and the configuration of a molding product.
There are a case in which the plasticizing and measuring process of the resin material is conducted in a state where a nozzle disposed on a tip of the heating cylinder is in contact with the fixed die (called "nozzle touch"), and a case in which the plasticizing and measuring process is carried out with the nozzle being moved backward so as to be apart from the fixed die.
When the plasticizing and measuring process is carried out in the nozzle touch state in repeating the molding cycle, if the temperature of the fixed die is set to be lower than the temperature of the molten resin material, the heat of the nozzle is transmitted to the fixed die to lower the temperature of the nozzle, as a result, the material is solidified in the nozzle so that the nozzle is clogged. Also, if the temperature of the fixed die is set higher than the temperature of the molten resin material, the temperature of the nozzle rises due to the transmission of the heat from the fixed die, as a result of which the material is hardened in the nozzle so that the nozzle is clogged. In any case, it may be difficult to carry out the continuous molding.
In order to eliminate the above problems, it is normal that the nozzle is displaced backward and spaced from the fixed die every time the injection in the molding cycle is finished. However, in the case where the plasticizing and measuring process is carried out in a state where the nozzle is moved back and spaced from the fixed die, there arise such problems that the molten resin material leaks from the nozzle due to the back pressure of the screw to lack the amount of the measured molten resin, that the surroundings are stained with the molten resin material or that the resin material remains in excess. Also, it is likely to occur such problems that the tip of the nozzle is cooled whereby the molten and plasticized resin material is deteriorated in fluidity to interfere with the injection process, or the physical properties of the molding product become uniform.
As the conventional prior art for coping with the above problems, there has been known, as disclosed in Japanese Utility Model Laid-Open Publication No. Hei 4-107015, an injection molding machine which moves an injection unit forward and backward to bring the injection nozzle in contact with and apart from the fixed die, and also which includes means for stopping the injection nozzle at a closed position and a closing member that renders the tip opening of the nozzle close. In this injection molding machine, the tip opening of the nozzle which is moved backward and spaced from the fixed die and stopped at the closed position is closed by the closing member during the measuring process, so that the resin material is prevented from leaking from the nozzle even if the screw is moved backward while applying a back pressure to the screw.
Also, as another prior art, there has been known, as disclosed in Japanese Utility Model Laid-Open Publication No. Hei 3-64724, in a heating device for an injection molding nozzle which makes the nozzle contact with the die at every injection molding, the nozzle tip portion is surrounded and heated by a heat retaining ring while the die is apart from the nozzle and the heat retaining ring is made apart from the nozzle tip portion while the die is in contact with the nozzle. Since this heating device is designed in such a manner that the heat retaining ring is automatically set to the nozzle tip portion while the die is opened and a product is taken out from the die after injection molding, and the heat retaining ring surrounds and rapidly heats the nozzle tip, and at the time of injection the heat retaining ring is separated and moved backward from the nozzle tip immediately before the nozzle tip is brought in contact with the die, there is no occurrence in which the molten resin is semi-solidified or partially solidified in a nozzle passage so that the melting state can be kept for the succeeding injection, and the nozzle tip can be prevented from being cooled.
However, in the device disclosed in Japanese Utility Model Laid-Open Publication No. Hei 4-107015 of the above-described prior art the means for making the injection nozzle contact to and leave apart from the fixed die requires also a function for stopping the injection nozzle at the closed position, and that, since the closing member is set in a state as a cantilever, the tip opening of the nozzle can merely contact to the closing member at the closed position, as a result there has been a problem in which the resin material cannot be surely prevented from leaking from the nozzle during the plasticizing and measuring process. Also, there arises another problem, in which, since the closing member contacts to the nozzle, the heat of the nozzle is transmitted to the closing member and the temperature of the nozzle is lowered, whereby the resin material is solidified or hardened in the nozzle to cause the nozzle is to be clogged, thereby a continuous molding cannot be carried out. Further, because the device is of the type to stop the injection nozzle at the closed position, there arises another problem in which it takes time to make the injection nozzle contact to or leave apart from the fixed die from the stopped state, and as a result the efficiency per molding cycle cannot be attained.
Also, in the device disclosed in Japanese Utility Model Laid-Open Publication No. Hei 3-64724 of the above-described prior arts, while the nozzle leaving apart from the die, the tip of the nozzle is merely surrounded and heated by the heat retaining ring, in addition, the division-type heat retaining ring is used for separating and moving backward the heat retaining ring from the nozzle tip immediately before the nozzle tip contacts to the die, and which does not contain the technical idea that the nozzle is closed, so that a problem that the material kept in a melt state leaks from the nozzle has not been solved.
Further, the heat retaining ring disclosed in Japanese Utility Model Laid-Open Publication No. Hei 3-64724 and the closing member disclosed in Japanese Utility Model Laid-Open Publication No. Hei 4-107015 have a problem that foreign materials are stuck to them due to drooling and the like at the time of the nozzle touch and interposed between the nozzle and those members to cause malfunction, thereby disabling continuous molding.